The invention pertains to a device for determining the north direction comprising a wire-suspended gyro, a pickoff responding to deviations of the gyro from the zero position of the wire suspension, a torquer fixed to the gyro housing to which the highly-amplified pickoff signal is applied and which exerts on the gyro, about the suspension wire axis, a torque counteracting the gyro directing torque, and a measuring and indicating device for the deviation of the gyro spin axis from the north direction, sensitive to the amplified pickoff signal.
A wire-suspended gyro has a gyro directing torque tending to align the spin axis of the gyro to north. The transition of the gyro to the north direction requires much time. It is known to pick off the gyro deviations from the zero position of the wire suspension by means of a photoelectric pickoff and to apply the highly-amplified pickoff signal to a torquer which exerts a torque on the gyro housing about the suspension wire axis to counteract the deviation. A torque counteracting the gyro directing torque is therefore generated via the torquer and the gyro is thus caged to the zero position of the wire suspension when the gyro is about to drift from the zero position of the wire suspension under the influence of the gyro directing torque. The gyro directing torque is thus compensated by a countertorque. With an appropriate prealignment of the gyro and adequate amplification of the pickoff signal, this countertorque is then proportional to the angle formed by the zero position of the wire suspension and the north direction, in the same way as the gyro directing torque. As the torque is also proportional to the pickoff signal in accordance with the characteristic of the torquer, this signal is likewise proportional to the angle formed by the zero position of the wire suspension and the north direction.
Wire-suspended gyros are sensitive to vibrations. Difficulties are therefore encountered when using a device of the above mentioned kind on a base plate subject to vibrations, for example on a motor driven land vehicle. It could be tried to eliminate the disturbances by filtering the output signal, but such a filtering by means of conventional filters would again result in an unsatisfactory lengthening of the measuring period. In most instances it is important to obtain the indication of the north direction as quickly as possible.
The basic purpose of the invention is to provide a device for determining the north direction which can be used on supports subjected to vibrations, and also which quickly furnishes an indication of the north direction.
A prior invention (West German patent application No. P 25 45 026. 4; filed Oct. 8, 1975; entitled "Device For Determining North Direction; Uwe Krogmann, inventor) proposed a solution in which the measuring and indicating device included an analogue-digital converter for generating a digital signal proportional to the pickoff signal and a digital filter for the recurrent averaging of the digital signals sampled during the preset time intervals and for the immediate output of the last average value formed each time. By means of this averaging, any interferences of the measuring signal produced by vibrations were averaged out. By having the averaging recurrent, an indication of the north direction -- possibly still faulty -- was made quickly available, and then continuously improved. The average value was formed continuously taking into account all previous measured values by using the following recurrence formula EQU U.sub.M (t.sub.n) = U.sub.M (t.sub.n-1) + EQU K.sub.n [U.sub.M (t.sub.n) - U.sub.M (t.sub.n- 1)]
where U.sub.M (t.sub.n) is the instantaneous measured value sampled at the time t.sub.n and U.sub.M (t.sub.n) the average value of all previous measured values sampled up to the time t.sub.n. However, the cost to produce a digital filter operating according to the above mentioned recurrence formula is relatively high.
The basic purpose of the present invention is to provide a device of the type of the previous invention embodying a simplified digital filter without substantially reducing the filtering action and to further decrease the measuring time for the determination of the north direction. This is achieved by having the digital filter, which receives the digital signals sampled at time intervals T, produce an output signal in accordance with the equation: EQU U.sub.M (nT) = U.sub.M ((n-1) .multidot. T) + EQU 1/N [U.sub.M (nT) - U.sub.M ((n-N) .multidot. T)) ]
where
n is a consecutive whole number, PA1 U.sub.M (jT) the average value output at the time jT, PA1 u.sub.m (jT) the digitalized measuring value at the time jT, and PA1 N a fixed whole number.
In accordance with the invention, the digital filter continuously forms an average value. Only the last N measuring values are however considered in each case for this average value formation. A "value storage window" for N measuring values each time is displaced in a way on the time axis via the transient curves of the meridian gyro, and the average value of these N measuring values is formed recurrently.
In the recurrence formula on which the operation of the digital filter is based according to the previous invention supra, the expression in brackets with which the previous average value U.sub.M (t.sub.n-1) must be corrected to obtain the actual average value U.sub.M (t.sub.n) includes the previous average value U.sub.M (t.sub.n-1). This average value U.sub.M (t.sub.n-1) therefore appears twice in the recurrence formula. The factor K.sub.n is time-controlled. For this reason it has to be computed either "on-line" or "off-line", and the K.sub.n factors are made available in a memory. The digital filter according to that invention is therefore rather elaborate.
In contrast to this the filter according to the present invention utilizes in the expression in brackets to be provided as correction for the previous average value U.sub.M ((n-1) T), only the instantaneous measuring values U.sub.M (nT) and U.sub.M ((n - N) T), hence not even one average value. The factor 1/N before the bracket is a constant and corresponds practically to the reciprocal of the length of the "window" on which the averaging is based. It will therefore be seen that such a digital filter can be of simpler design than the filter according to the previous invention. It has been shown by means of a simulation performed with a digital computer for the purpose of investigating the response of the filter in accordance with the present invention to the effect of white noise and a slow sinusoidal disturbance that a suitable indication of the north direction can be obtained in a relatively short time with the digital filter in accordance with the invention.